A. Field of the Invention
The present invention relates to absorbing, anti-reflection (AR) coatings.
B. Description of the Related Technology
Conventional absorbing anti-reflection coatings (absorbing ARs) comprise multiple pairs of thin metal underlayers and approximately quarter-wave thick low index dielectric overcoats. The coating terminates in a low index dielectric layer. This design approach is described in Joseph H. Apfel, "Graphics in Optical Coating Design," Applied Optics, Vol. 11, p. 1303, 1972.
Using the above design philosophy, one can optimize for minimum reflection, ignoring transmission. The result is a so-called "dark mirror". Typical applications of dark mirrors include the internal baffling of telescopes and other high gain optical systems. Alternately, one can compromise between intermediate levels of transmission and slightly higher reflection. This ability to select transmission and reflection has led to the perhaps overly simplistic description of absorbing AR's as being "tunable."
The assignee, Optical Coating Laboratory, Inc. has marketed a two period absorbing AR coating deposited on PET for use in GlareGuard.RTM. contrast enhancement filters. This absorbing AR product used a four layer nickel/magnesium fluoride design: Ni/MgF.sub.2 /Ni/MgF.sub.2. A theoretical plot of this design is depicted in FIG. 1, for comparison to the performance of product embodying the present invention. This prior product is characterized by conductivity (sheet resistivity) of about 2000 .OMEGA./.quadrature., coating transmission .ltoreq.60%, and reflection .ltoreq.0.25%. The nickel/fluoride coating can not be sputtered successfully.
A central point of the Apfel teaching, and of subsequent designs, is that coatings employing absorbing (metallic) materials can provide low reflectance performance over a broad band by employing multiple periods of metal/dielectric layers. In general, absorbing AR coatings for display applications have been limited to one or two periods, apparently in an attempt to balance the requirement of low reflection against the competing requirement of high transmission, which requires minimizing the metal content of the coating. For many applications, establishing this balance with 40%-60% transmission is adequate. It seems impossible to construct the repeating period structure with metal films which are thin enough to produce higher transmission.
However, some contrast enhancement applications do require a higher level of transmission, such as 70%. Such a high level of transmission applies where it is necessary or desirable to match as closely as possible the optical performance of current optical system designs which use an all dielectric anti-reflection coating applied to absorbing glass. For example, replacing the 70% transmitting bonded safety panel on a CRT with a coating directly deposited onto the CRT faceplate may require that the coating likewise exhibit 70% transmission.
To achieve such a high level of transmission performance, it is necessary to decrease the thickness of the metal to the extent that other limiting factors become operative (dominant). For example, as films get progressively thinner, they begin to lose their electrical conductivity, either through anomalous skin effects or because they are so thin as to be discontinuous, that is, they do not form a continuous film.